A fluorescent lamp is basically a glass tube filled with a gas, such as a combination of neon and a small amount of mercury vapor. The interior of the tube is coated with a phosphorus material and each end of the tube includes a filament cathode and an anode structure. In operation, each end of the tube is alternately the anode or the cathode during one half of the alternating current cycle.
When a high voltage, on the order of several hundred volts, is established between the two ends of the lamp, the gas within the tube becomes ionized and forms a conduction path, thereby producing an electric arc through the gas. After the gas is ionized and an arc is formed, the lamp has an extremely low electrical resistance. The electric current passing through the lamp produces energized molecules and electrons which strike the phosphorus material which then produces light that is emitted from the tube.
During operation of the lamp, the anode serves as the collector for charged ions. Heat is generated at the anode by the bombardment of arriving ions on the anode. The amount of heat generated by the arriving ions is determined by the relative anode voltage and the length of time the anode is positively charged. Thus, low frequency alternating current, such as standard 60 hertz, causes the anode to collect ions from a great distance because it is positively charged for a relatively long time. The ions accelerate toward the anode during the entire half cycle, and the ions farthest from the anode arrive at relatively high velocities, imparting significant mechanical energy to the anode. The energy of ion bombardment causes heating and erosion of the anode. The erosion of the anode is a major factor affecting the lifetime of the lamp and a major limitation to the maximum light intensity that can be obtained from a given fluorescent lamp.
The power and the lifetime of a fluorescent lamp are affected by the frequency of the alternating current and the shape, or "crest factor", of the alternating current waveform. In any given waveform there is a peak voltage and an average voltage. Although a certain minimum voltage is necessary to operate a fluorescent lamp, the ideal waveform is a square wave, which has the lowest ratio of peak to average voltage, or the lowest crest factor. The square wave produces the highest average current with the least amount of anode erosion caused by high peak voltage. Other waveforms can provide the same average current, but with an undesirable high peak voltage that produces a current pulse during the cycle. During the current pulse, ions arrive at the anode with greater energy, causing rapid erosion of the electrodes and limiting power and efficiency of the lamp.
Prior art ballast circuits have not been designed to maximize the lifetime of fluorescent lamp electrodes in operations involving either low power dimming or high light intensity. Prior ballast circuits generally provide an undesirable distribution of output energy with respect to time, either in the waveform shape, the time intervals between voltage pulses, or both. Ballast circuits which provide for lamp dimming by increasing the time period between high power voltage pulses cause disproportionate anode erosion in relation to the low light intensity produced. Ballast circuits which provide for lamp dimming by changing the waveform shape of a fixed frequency alternating current produce a high crest factor which causes disproportionate electrode erosion during the high power pulse, thereby limiting the life of the lamp and the usable dimming range.
In general, prior art ballast circuits do not provide for optimum lamp life in either dimming operations or high intensity operations. Therefore, there is a need for a fluorescent lamp ballast circuit which provides extended lamp lifetime by minimizing electrode erosion during lamp start-up, dimming operations, and high intensity operations.